This project is aimed at understanding the mechanisms of sex- dependent gene regulation of steroid hydroxylases in liver and kidney. Full-length cDNAs encoding for three sex-specific hepatic testosterone hydroxylases in mice were isolated and sequenced. With the use of the cDNAs, the following have been uncovered. I-P-450 16 alpha (female-specific 16 alpha- hydroxylase): The specific expression in females in regulated by a single locus named Rip, and its repression in males is under the control of the locus named Ripr. Both loci are closely localized on chromosome 7, along with the I-P-450 16 alpha structural gene which is greater than 35kb in size and has 9 exons. It was found that growth hormone and estrogen are repressors of I-P-450 16 alpha in males. Genetic evidence suggests that estrogen acts on Ripr locus. C-P-450 16 alpha (male-specific 16 alpha- hydroxylase): The comparisons of mRNA and activity levels in livers from androgen-treated 129/J mice, which had been castrated at day one or at adulthood, provided the evidence that there are at least two differentially regulated C-P-450 16 alpha's in livers from adult males; one is neonatally imprinted and the other is reversibly regulated by androgen. The expression of C-P- 450 16 alpha in males is also regulated by growth hormone. It is, therefore, interesting to see how the pituitary and sex hormones are cooperating to regulate this hydroxylase gene. The cDNA of C-P-450 16 alpha was ligated to pcD vector and transfected in COS cells, resulting in expression of 16 alpha-hydroxylase activity. The gene for reversibly regulated C-P-450 16 alpha was characterized to be about 5kb with nine exons. P-450 15 alpha (female-specific 15 alpha-hydroxylase): Two highly homologous, but differentially regulated genes (Types I and II) were identified and isolated. The type I gene is predominantly expressed in female but not male livers, but expressed only in male but not female kidney. The repression of P-450 15 alpha in male liver and the expression in male kidney are reciprocally regulated by androgen. Type II genes are female-specific in both liver and kidney, although the level of expression in kidney is minimal.